1. Field of the Invention
The present invention relates to a method of producing small cylindrical products, especially metallic products such as bearing races and valve spool elements, from cylindrical bar-like and individual workpieces held on two separate rotatable spindles of a bi-spindle machine tool, by employing two separate groups of cutting tools mounted on a single tool block unit, the movement of which is controlled by a numerical controller in accordance with a numerical control program. The present invention also relates to a bi-spindle numerically controlled machine tool used for carrying out the above method.
2. Description of the Related Art
Generally, there are two methods of producing small cylindrical metallic products such as inner and outer races of bearings and valve spools from cylindrical workpieces.
In the first method, separate manufacturing processes used in the machining of each product are performed by employing individual specific mono-functional machine tools, and therefore, a number of such specific mono-functional machine tools must be prepared to produce one product; which requires a large floor space in a factory for the installation of the many machine tools, and many operators to control the operations of the separate mono-functional machine tools, and as a result, the manufacturing costs for each product become very high. Further, it is difficult to maintain a high accuracy when machining the products, due to the employment of many mono-functional machine tools, and accordingly, the production yield of accurate products is low. Consequently, a strict and expensive quality control of the products during the machining thereof by each of the mono-functional machine tools is necessitated. Moreover, loading and unloading the workpieces when moving the workpieces among the respective mono-functional machine tools often causes damage to the products.
In the second method, many numerically controlled machine tools, each capable of accomplishing a plurality of machining processes necessary for producing each of such products, are employed to carry out a mass production of identical products.
In connection with the second method, the assignee company of the present application (Seibu Denki Co., Ltd. of Japan) has developed a numerically controlled lathe provided with a single spindle having a chucking device for chucking a workpiece and capable of rotating at a high speed, and a single tool block capable of moving left and right and back and forth with respect to the rotating spindle, to cut the workpiece by using a plurality of cutting tools mounted on the tool block. The movement of the tool block is controlled by a numerical controller in accordance with a numerical control program, and thus the numerically controlled lathe is able to continuously machine each workpiece in accordance with the numerical control program until a complete product is automatically obtained.
The same assignee company has also developed a bar feeder capable of feeding a long cylindrical workpiece (a bar-shape workpiece) in the spindle of the numerically controlled lathe. The bar-shape workpiece is held by the front end of the spindle without using the chucking device and formed into a product having a desired shape and size by cutting tools mounted on the tool block. After completion of the machining process, the product is cut from an end of the bar-shape workpiece by a cutting-off tool provided on the tool block. Nevertheless, since each product must be machined from the beginning to the end by one numerically controlled lathe, it takes a long time to obtain each completed product, and therefore, a plurality of identical numerically controlled machines must be arranged for producing many products during a unit of time, and accordingly, a wide floor area is required for a factory for the installation of the many numerically controlled machines, which is very expensive. Consequently, an effective reduction in manufacturing cost for each product cannot be achieved.
On the other hand, when both front and rear sides of a small workpiece must be sequentially machined during the production of a small metallic product by conventional machine tools such as conventional engine lathes, a pair of machine tools arranged in a face to face relationship are employed so that each machine tool cuts one of the front or rear sides of the workpiece. In this case, a complicated handling device must be always provided between the pair of machine tools, to move each workpiece from one to the other of the machine tools. This brings drawbacks such that a sufficient floor space must be prepared for the installation of the pair of machine tools and the workpiece handling device, and that the cost of manufacturing one product by using these machine tools and the complicated handling device is necessarily high.
To obviate these drawbacks, a single lathe having a pair of confronting spindles and a workpiece handling device able to be moved between the spindles to carry a workpiece from one to the other spindle, and vice versa, has been developed. FIG. 36 illustrates an important portion of the developed lathe, which is provided with two axially confronting spindles Ha and Hb capable of holding a workpiece W, respectively, and a handling device for moving the workpiece W from one spindle to the other spindle. The handling device has two independent rotatable arms Aa and Ab mounted on an axial slide E slidably movable between two conveyers Ca and Cb. The arms Aa and Ab catch a workpiece W before machining from the conveyer Ca, attach the workpiece W before machining to one of the spindles (the spindle Ha), move the workpiece W after machining at the spindle Ha to the other spindle Hb, remove the workpiece W from the spindle Hb after completion of the machining at the spindle Hb, and return the machined workpiece W to the other conveyor Cb. Nevertheless, a drawback arises in that, since the starting and finishing times for machining of a workpiece on the spindle Ha are often different from those for machining of the workpiece on the spindle Hb, a complicated sequential control program must be prepared to determine the operations of the spindles, tool carriages, and the handling device. Moreover, the complicated sequential control program must be changed in response to a change in products, and this is very inconvenient.